Automatic tool holder clamping units are widely used today. One of the most versitile and efficient clamping unit designs entails the utilization of a movable lock rod. In such a design, the lock rod, through a locking ramp formed on the surface thereof engages a pair of locking balls. As the locking rod is moved from an unlocked position to a locked position, the locking ramp engages the locking balls and urges them outwardly where they project through selected apertures formed in the clamping unit and engage the shank portion of the tool holder. The locking rod is loaded such that it transmits through the locking balls a radial or perpendicular (relative to the longitudinal axis of the locking rod) locking load against the shank of the tool holder.
There are basically two types of automatic clamping units that utilize this lock rod concept. First, there is the torque nut design which utilizes a nut threaded onto a draw rod that is connected to or forms a part of the lock rod. The particular load placed on the lock rod can be varied and particularly set by the amount of torque applied to the nut.
The second type of lock rod automatic clamping unit is the spring type. This approach utilizes a series of disk springs that form an integral part of the clamping unit and which are disposed such that they engage the lock rod and bias the same towards a locked position.
The disk spring or variable force-type clamping unit is a very popular and desirable clamping unit because of its simplicity. However, it is more difficult to control the resultant locking forces in the disk spring-type automatic clamping unit. This is because the position that the lock rod assumes in the lock position may vary due to manufacturing tolerances. Because of the nature of springs, the force applied to the lock rod and ultimately transferred outwardly for purposes of locking will vary due to the position that the lock rod assumes in the locked position. This is particularly evident in high mechanical advantage designs.
Presently, the locking angle of the locking ramp formed on the lock rod has been formed by a surface inclined at a set angle. Therefore, the mechanical advantage applied by the locking rod is constant throughout the stroke of the locking rod. Therefore, the resultant outward locking force exerted by the locking elements or balls engaged with the constant angle locking ramp will vary depending on the position of the locking rod and the forces applied against the locking rod by the disk springs. Obviously, a significant variation in locking forces affects the performance of the cutting tool and the tool holder.